1. Field of the Invention
The invention relates to apparatus for producing a composite video signal from digital colour difference and luminance signals.
2. Description of The Prior Art
In a conventional encoder the digital colour difference (R-Y and B-Y) signals are demultiplexed and applied to digital to analogue converters (DACs). The outputs of the DACs are combined with the burst gate signal in respective combining circuits whose outputs are fed to the signal inputs of respective modulators. The colour subcarrier signal is fed to a quadrature phase generator to produce two output signals differing in phase from each other by 90.degree., these output signals being fed to the carrier inputs of the respective modulators. The signal fed to the R-Y modulator is fed through a switchable phase inversion circuit which is switched to invert the phase on alternate lines. The outputs of the modulators are fed to a combining circuit together with the luminance signal which is also converted in a DAC to an analogue signal. The combined signal is subsequently blanked and the synchronising pulse are added to produce the composite signal.
With this system extreme care has to be taken in the design of the circuits for producing the phase inversion of the colour subcarrier and in accurately blanking the video signal. The circuits usually used for performing both these functions are based on the use of balanced modulators. The phase inversion produced is usually accurate in phase but the amplitude is not so accurately controlled and produces a resultant phase error after encoding. This phase error produces a visible effect on display known as "twitter". Blanking requires a shaping filter and careful design of the blanking modulator to maintain a good blanked state and to remove spikes or out-of-balance effects during the transitions between the unblanked and blanked states. Consequently these circuits tend to be complicated and expensive to produce.
It is to be noted that the foregoing applies, strictly speaking, for the
colour television system but that similar difficulties also arise with other systems.